This invention relates generally to vacuum die casting and more particularly to a die-casting method in which, by controlling the degree of vacuum in the cavity within the casting mold within a specific range, formation of blowholes in the casting due to gas being swept into the cavity during injection thereinto of molten metal to be cast is prevented, thereby contributing to stabilization of the product quality of the cast articles. The invention also relates to a die-casting apparatus for practicing this method.
The principal component of a vacuum die-casting machine, in general, is a metal die mold having a cavity therein for forming a cast product or casting. The cavity is connected at its one end via a vacuum valve and a vacuum piping to a vacuum creating means. A vacuum gage is installed in the vacuum piping to indicate visually the degree of vacuum therein. The degree of vacuum within the cavity is controllable by the opening and closing of the vacuum valve. The cavity communicates at an opposite end thereof to the inner end of an injection sleeve. An injection plunger is slidably fitted in the sleeve and is operable therewith to inject molten metal or melt, to be die cast, into the cavity. The injection plunger has a rod extending out of the sleeve and being drivable by driving means. Detection devices such as limit switches are provided to detect certain critical positions of the rod and to thereby operate a relay for opening and closing the vacuum valve.
In the operation of this vacuum die-casting machine, the plunger is initially at its fully retracted position. Molten metal or melt is poured into the injection sleeve through a melt inlet formed in the sleeve. The plunger is then driven forward to begin injecting the melt into the cavity and close the melt inlet. One detection device is thereupon activated by the rod and, in turn, activates the relay. The relay thereby operates to energize a solenoid to open the vacuum valve. Thus reduction of the pressure within the cavity begins while the plunger continues to advance further, causing the melt to fill the cavity. Immediately before the cavity is completely filled with the melt, a second detection device is activated by the plunger rod, whereby the relay operates to close the vacuum valve. In this manner the melt is forced to rapidly fill the cavity in a state of amply reduced pressure. The structure composition and operation of a typical example of such a vacuum die-casting machine and its operating control means will be described in detail hereinafter in conjunction with a drawing.
In the control means of the prior art for operating a vacuum die-casting machine, the timing of the opening and closing of the vacuum valve is interlocked with only the mechanical movement of the rod of the injection plunger and is independent of the supervisory control of the degree of vacuum in the cavity. For this reason there is the possibility of gas being sucked into the cavity as it is swept in together with the melt. More specifically, in a case such as that wherein the vacuum valve is opened prematurely when the liquid surface of the melt within the injection sleeve is lower than the centerline of the sleeve, a gap which is not sealed by a portion of the melt exists between the inner wall surface of the sleeve and the outer peripheral surface of the plunger. Consequently, a large quantity of outside air flows through this gap into the interior of the sleeve. Thus, as melt is sucked into the cavity, it entraps and sweeps this air into the cavity. This air-infiltration phenomenon gives rise to the formation of cavities or blowholes in the resulting casting. Thus it becomes a cause of degradation of the product quality.
Furthermore, the timing of the opening and closing of the vacuum valve is established by only the positional relationships between the detection devices and the rod of the injection plunger. For this reason, adjustment of this timing cannot be carried out during the operation of the die-casting machine.
In addition to this timing, the supervisory control of the degree of vacuum applied to the cavity is also an important factor for stabilizing the quality of the cast products. Heretofore, however, control of this timing and control of the degree of vacuum have been carried out separately and independently. Especially with respect to control of the degree of vacuum, this control has been carried out exclusively by visual supervision with the use of a vacuum gage.